1. Field of the Invention
The present invention is directed to managing an out of available space condition.
2. Description of the Related Art
In prior art virtual tape storage systems, hard disk drive storage emulates tape drives and tape cartridges. For instance, host systems perform input/output (I/O) operations with respect to a tape library by performing I/O operations with respect to a set of hard disk drives that emulate the tape library. In prior art virtual tape storage systems, such as the International Business Machines (IBM) Magstar** Virtual Tape Server, at least one virtual tape server (VTS) is coupled to a tape library comprising numerous tape drives and tape cartridges. The VTS is also coupled to a direct access storage device (DASD), comprised of numerous interconnected hard disk drives. **IBM, Tivoli, RS/6000, AIX, and Magstar, OS/390 are trademarks of International Business Machines Corp.
The DASD functions as a cache to volumes in the tape library. In VTS operations, the VTS receives the host's requests to access a volume in the tape library and returns data for such requests, if possible, from the cache. If the volume is not in the cache, then the VTS recalls the volume from the tape library to the cache, i.e., the VTS transfers data from the tape library to the cache. The VTS can respond to host requests for volumes that are present in the cache substantially faster than requests for volumes that have to be recalled from the tape library to the cache.
Since the cache can satisfy requests faster than the tape library, I/O requests can be satisfied faster if frequently accessed volumes are kept in the cache. However, since the capacity of the cache is relatively small when compared to the tape library, not all volumes can be kept in the cache. Hence, the VTS also premigrates volumes from the cache to the tape library, i.e., the VTS transfers data from the cache to the tape cartridges in the tape library. The process of transferring data from the cache to the tape cartridges is referred to as premigration. Eventually, these premigrated volumes will be removed from the cache and shortened to a pointer to the data on tape cartridges, freeing space in the cache for new data. This shortening, or “migration,” operation is very fast, and the performance bottleneck in the VTS is the premigration operation.
The tape cartridges in the tape library may be organized as pools of tape cartridges. That is, a pool is a set of tape cartridges. The host identifies the pool to which particular data in DASD is written. When there is no unused space on tape cartridges available in a pool to which data from DASD is to be written, it is possible that data for pools that are do not have unused space on tape cartridges continue to write to DASD. Since there is no unused space on the tape cartridges to transfer data for these pools out of DASD, data for these pools eventually may consume all space in DASD.
Thus, there is a need in the art for improved techniques for managing premigration of data.